EP0835947B1 - Aluminized sheet with poor emissivity and process for making the same - Google Patents

Abstract

A steel sheet is coated on at least one side with an aluminium-based alloy containing up to 11% by weight of silicon, which gives it an emissivity below 0.15 for wavelengths between 1.5 and 15 microns. Also claimed are: (i) making the coated sheet, heating the coating above its melting point for 0-100, preferably 0-10 secs. and cooling to below the temperature at which the steel and alloy bond;, and (ii) a thermal screen including the sheet.

Description

The present invention relates to the field of sheets aluminized.

It relates specifically to aluminized sheets whose layer coating consists of an aluminum-silicon alloy, used by example for making thermal screens of exhaust lines of motor vehicles.

The purpose of a heat shield is to isolate the parts located behind him the heat source in front of him. So a screen must be able to absorb as little energy as possible, or in other words, to return the maximum. This translates into low emissivity of the constituent material, or in other words, reflectivity high.

The heat shields are therefore made of materials which, on the one hand, have sufficient mechanical characteristics, a good formability, good corrosion resistance, and other apart from low emissivity.

It is known to produce heat shields from sheets aluminized whose coating layer consists of an alloy aluminum-silicon.

Such a sheet is for example a sheet of mild steel, coated on its two main faces of an aluminum-silicon alloy by passing through soaked in a molten bath of said alloy.

When the sheet passes through the aluminizing bath, there is development of a layer of iron-aluminum-silicon alloy.

• une couche de surface de composition voisine de celle du bain,
• une couche sous-jascente d'alliage ternaire, ayant la composition suivante Fe₃Si₂Al₁₂.

Therefore, the coating has, in metallographic section, the following structure:

• a surface layer with a composition close to that of the bath,
• an underlying layer of ternary alloy, having the following composition Fe ₃ Si ₂ Al ₁₂ .

These aluminized sheets have a low total emissivity, less than 0.2, and therefore a high reflectivity, greater than 80%.

This characteristic is maintained up to 450 ° C.

This material is therefore very interesting and widely used for walls interior of industrial or domestic ovens, heat reflectors on all household heaters, or to make heat shields for parties the warmest of the exhaust systems of motor vehicles.

It is known to improve the properties of this material by a pass in a hardening cage, called "skin-pass" with smooth cylinders, but if this improvement makes it possible to slightly decrease the material's remissivity, it does not allow keep its properties for use at very high temperatures.

Document WO 85/00386 describes a treatment for alloying a steel aluminized, at a high temperature for a very long time.

Document JP 55085623 describes an aging treatment for a aluminized steel, at a temperature below the coating melting temperature.

The present invention aims to solve this handicap by having for object an aluminized sheet of which the coating layer consists of an alloy aluminum-silicon, with a low emissivity and usable as thermal screens heat sources with a temperature above 500 ° C, such as for example hottest parts of motor vehicle exhaust systems.

The invention relates more particularly to a steel sheet coated on at least one of its main faces with a layer of a coating made of an alloy with aluminum base comprising aluminum and silicon, with weight percent less than 11% silicon, essentially of the type comprising in weight percent between 7 and 11% silicon and between 87 and 93% aluminum, characterized in that the coated side has a monochromatic emissivity of less than 0.15 for all lengths waves between 1.5 and 15 micrometers.

According to another characteristic, the coated face has an emissivity monochromatic less than 0.10 for all wavelengths between 5 and 15 micrometers, and a monochromatic emissivity between 0.10 and 0.15 for all wavelengths between 1.5 and 5 micrometers.

• élaboration d'une tôle d'acier revêtue sur au moins une de ses faces principales d'une couche d'un revêtement à l'état solide, constitué d'un alliage à base d'aluminium comportant de l'aluminium et du silicium, avec en pour-cent pondéraux moins de 11 % de silicium, du type comportant en pour-cent pondéraux entre 7 et 11 % de silicium et entre 87 et 93 % d'aluminium,
• chauffage de la couche de revêtement jusqu'à une température T1, supérieure à la température T2 de fusion dudit revêtement,
• maintien de la couche de revêtement à ce niveau de température supérieure à la température de fusion du revêtement pendant une durée comprise entre 0 et 100 secondes, de préférence entre 0 et 10 secondes,
• refroidissement de la tôle jusqu'à une température au moins égale à la température de fin d'alliation entre le revêtement et l'acier, de préférence jusqu'à la température ambiante.

The invention also relates to a method of manufacturing such a steel sheet, characterized in that it comprises the following steps:

• production of a steel sheet coated on at least one of its main faces with a layer of a coating in the solid state, consisting of an aluminum-based alloy comprising aluminum and silicon, with in weight percent less than 11% of silicon, of the type comprising in weight percent between 7 and 11% of silicon and between 87 and 93% of aluminum,
• heating the coating layer to a temperature T1, higher than the melting temperature T2 of said coating,
• maintaining the coating layer at this temperature level above the melting temperature of the coating for a period of between 0 and 100 seconds, preferably between 0 and 10 seconds,
• cooling of the sheet to a temperature at least equal to the temperature at the end of the alloy between the coating and the steel, preferably to room temperature.

• la température de chauffage T1 est comprise entre la température de fusion de la couche de revêtement et 650°C ;
• la température T1 est supérieure, entre 10 et 15°C, à la température de fusion de la couche de revêtement ;
• le chauffage de la couche de revêtement est effectué à une vitesse comprise entre 20 et 100°C par seconde ;
• le refroidissement de la tôle est un refroidissement naturel à l'air libre, ou un refroidissement forcé par rayonnement ;
• le refroidissement de la tôle est un refroidissement forcé à l'air ;
• le refroidissement de la tôle s'effectue en au moins deux étapes comprenant :

• un refroidissement naturel jusqu'à la température T2 de fusion du revêtement,
• puis un refroidissement forcé à l'air jusqu'à la température de fin d'alliation entre le revêtement et l'acier ;
• la tôle d'acier revêtue sur au moins une de ses faces principales d'une couche d'un revêtement à l'état solide, constitué d'un alliage à base d'aluminium, du type comportant de l'aluminium et du silicium, avec en pour-cent pondéraux moins de 11 % de silicium, est élaborée par trempage d'un substrat d'acier dans un bain en fusion contenant entre 9 et 10 % de silicium, environ 3 % de fer, le reste étant de l'aluminium, et refroidissement jusqu'à une température inférieure à la température de fusion du revêtement.

According to other characteristics:

• the heating temperature T1 is between the melting temperature of the coating layer and 650 ° C;
• the temperature T1 is higher, between 10 and 15 ° C, than the melting temperature of the coating layer;
• the heating of the coating layer is carried out at a speed of between 20 and 100 ° C per second;
• the cooling of the sheet is natural cooling in the open air, or forced cooling by radiation;
• the cooling of the sheet is forced air cooling;
• the cooling of the sheet is carried out in at least two stages comprising:

• natural cooling to the coating melting temperature T2,
• then forced air cooling to the end of alloy temperature between the coating and the steel;
• the steel sheet coated on at least one of its main faces with a layer of a coating in the solid state, consisting of an aluminum-based alloy, of the type comprising aluminum and silicon, with in weight percent less than 11% of silicon, is produced by dipping a steel substrate in a molten bath containing between 9 and 10% of silicon, approximately 3% of iron, the rest being aluminum, and cooling to a temperature below the coating melting temperature.

Finally, the invention also relates to a heat shield made from such a sheet.

• la figure 1 est une courbe représentant l'émissivité spectrale d'une tôle aluminiée B selon l'invention, et d'une tôle aluminiée A de l'état de la technique ;
• les figures 2 et 3 sont des courbes représentant l'effet du chauffage d'une tôle aluminiée selon l'invention sur son émissivité.

The characteristics and advantages will appear more clearly after the description which follows, given solely by way of example, made with reference to the single sheet of drawings appended, on which:

• FIG. 1 is a curve representing the spectral emissivity of an aluminized sheet B according to the invention, and of an aluminized sheet A of the state of the art;
• Figures 2 and 3 are curves showing the effect of heating an aluminized sheet according to the invention on its emissivity.

As can be seen in Figure 1, the characteristic main of the aluminized sheet coated on at least one of its faces main layers of a coating made of a base alloy of aluminum, of the type comprising aluminum and silicon, with in percent by weight less than 11% of silicon, according to the invention, resides in the causes the coated side to have a monochromatic emissivity less than 0.15 for all wavelengths between 1.5 and 15 micrometers.

More specifically, the coated face has an emissivity monochromatic less than 0.10 for all wavelengths between 5 and 15 micrometers, and a monochromatic emissivity between 0.10 and 0.15 for all wavelengths between 1.5 and 5 micrometers.

The term monochromatic emissivity should be understood as being the ratio between the luminance of the material considered at a length wave, on the luminance of a black body at this same length wave, and at the same temperature.

Such an aluminized steel sheet according to the invention is manufactured in many stages.

A first step is to develop a coated steel sheet on at least one of its main faces with a layer of a coating with solid state, consisting of an aluminum-based alloy comprising aluminum and silicon, with in weight percent less than 11% of silicon, of the type comprising in weight percent between 7 and 11% of silicon and between 87 and 93% aluminum.

A second step is to heat the coating layer up to a temperature T1, higher than the melting temperature T2 of said coating.

It is necessary to understand by melting temperature T2 the temperature when the coating begins to melt. Indeed, a coating based aluminum, such as that described above, is in the form of aluminum dendrites with an interdendritic phase and a phase dentritic. The interdendritic phase melts at a temperature below the dendritic phase, and the temperature T2 in question is the melting point of this interdendritic phase.

In a third step, the coating layer is maintained at this temperature T1, or in any case higher than T2 for a period between 0 and 100 seconds, preferably of the order of 2 to 10 seconds.

Finally, the last step is to cool the sheet to a temperature at least equal to the end of alloy temperature between the coating and steel, and preferably up to a temperature equal to the ambient temperature.

This manufacturing process allows the coating to be remelted aluminized.

The production of coated steel sheet on at least one of its main faces of a layer of a coating in the solid state, consisting of a aluminum-silicon alloy, of the type for example comprising in percent by weight between 7 and 11% of silicon and between 87 and 93% of aluminum, corresponding to the first step of the process of the invention, can be performed by dipping a steel substrate in a molten bath containing between 9 and 10% silicon, approximately 3% iron, the rest being aluminum, and cooling to a temperature below the coating melting temperature.

It is very important that the aluminized steel sheet produced in the first step of the process has a coating layer in the state solid, i.e. it has been cooled to a temperature below the coating melting temperature.

Regardless, to get the specifications of emissivity of the sheet according to the invention, that this temperature is equal to the coating melting temperature minus a few degrees, for example minus 5 or 10 ° C, or equal to room temperature.

The temperature T1 reached by the sheet during heating carried out in the second stage of the process must imperatively be higher than the coating melting temperature T2, in order to ensure a reflow of the coating layer, to obtain the characteristics in emissivity of the sheet according to the invention.

Preferably, this temperature T1 is between the melting temperature of the coating layer and 650 ° C.

This limit at 650 ° C allows on the one hand to limit the cost of second stage, and, on the other hand, has a beneficial effect on limiting the phenomenon of alloy between the coating and the steel.

To ensure that the coating layer is remelted in any point, it is best to heat the sheet to a temperature T1 between the melting temperature T2 of the coating layer plus 10 ° C and the melting temperature T2 of the coating layer plus 15 ° C.

This characteristic makes it possible to get rid of possible phenomena of slight temperature heterogeneities due to example to heterogeneities in thickness of the coating layer, or to heating process implemented.

It is important that we reach this temperature quickly T1 in order to limit the phenomena of alloy between the coating and the steel of the substrate. Thus, the heating rate is advantageously between 20 and 100 ° C / second.

In the case where the temperature of the coating layer of the sheet produced during the first stage is close to the temperature T2 coating melting, you can choose a heating rate between 20 and 30 ° C / second, because in this case, it is not necessary to raise the temperature of the sheet only a few tens of degrees, of the order of 20 to 50 ° C.

On the other hand in the case where the temperature of the layer of coating of the sheet produced during the first stage is close to room temperature, a heating rate between 90 and 100 ° C / second, because in this case, the sheet temperature should not be raised only a few hundred degrees, on the order of 500 to 600 ° C.

The third step in the process is to maintain the layer coating at this temperature T1 for a period between 0 and 5 seconds.

It is possible to cool the sheet (last step of the process) immediately after the layer of coating has reached at any point a temperature T1 higher than the melting temperature of said coating.

For example in the case where the temperature T1 reached by the coating layer during the heating stage (second stage of the process) is between the melting temperature of the layer of coating plus 10 ° C and the melting temperature of the coating layer plus 15 ° C, it is quite possible not to provide a holding level at this temperature T1. But keeping the coating layer at this temperature T1 does not harm the invention insofar as this level of hold does not exceed one hundred seconds.

Indeed, the Applicant has realized that if we maintains this temperature T1 for a period greater than 100 seconds, the emissivity of the coating layer is increased too much to a substrate made of standard steel or titanium IF steel, the latter starting to grow from 10 seconds. In the case of renitrided steels, the appearance the alloying phenomenon being delayed due to the presence of nitrogen, the emissivity is not yet increased, but we note a surface finish oxidized, the aluminized sheet then having a whitish then yellowish appearance.

This phenomenon is perfectly visible in Figure 2 which represents the total emissivity curve of the coating layer in depending on its temperature.

This curve was developed from an aluminized sheet consisting of a titanium IF steel substrate with a thickness of 0.3 mm, coated a layer of a coating comprising 9.5% silicon, 3% iron, the rest being aluminum, thickness equal to 20 micrometers.

This aluminized sheet, at room temperature, was heated to bring the temperature T1 of the coating layer to 600 ° C., higher than the coating melting temperature T2, in this case 480 ° C in this example, and was maintained at 600 ° C.

During the entire heating phase and the maintenance phase 600 ° C, the total emissivity of the layer of coating for wavelengths between 1.5 and 14.5 micrometers using a spectroradiometer.

We can see very clearly on this curve that from the temperature of coating, the emissivity of said coating decreases, then after a ten seconds of holding at 600 ° C, it starts to grow again slowly, then faster from 100 seconds of holding at 600 ° C.

The Applicant also realized that this gradual increase in emissivity was only related to the duration of the maintaining the coating layer at temperature T1.

Indeed, as can be seen in Figure 2 (lines dotted lines), the fact of cooling the coating layer makes it possible to stop increasing the emissivity of the coating layer.

The curve shown in Figure 3 illustrates the effect known nitrogen on the coating alloy phenomenon.

This curve was developed from an aluminized sheet consisting of a renitrided steel substrate, having a nitrogen content higher than that of the previous titanium IF steel. The coating layer and the heat treatment performed are identical to the previous ones.

We can see very well on this curve, if we compare it to the curve in Figure 2, that the emissivity of the coating only starts to grow again from 120 seconds.

The last step of the process therefore consists in cooling the sheet up to a temperature at least equal to the end of alloy temperature between the coating and the steel, preferably up to room temperature.

This cooling can be a natural air cooling free, forced cooling by radiation, or a forced air cooling.

• un refroidissement naturel entre la température T1 et la température de fusion du revêtement,
• un refroidissement forcé à l'air entre la température de fusion du revêtement et la température de fin d'alliation entre le revêtement et l'acier.

Preferably, the cooling of the sheet is carried out in at least two stages comprising:

• natural cooling between the temperature T1 and the melting temperature of the coating,
• forced air cooling between the melting temperature of the coating and the end of alloying temperature between the coating and the steel.

It is preferable indeed, to avoid degrading the properties emissivity of the coating layer, to realize at first up to the coating melting temperature, cooling without contact with the coating layer still in the molten state.

Natural air cooling, or forced by radiation in passing the coating layer near a refrigerated wall, ideal for this first stage of cooling.

Forced cooling, for example with air, at least between the coating melting temperature and the end temperature between the coating and the steel, helps limit this phenomenon of alliance.

Plus heating / holding / cooling cycle is short, the better the aluminized sheet according to the invention, because we limit with a cycle runs the time that the aluminized sheet will spend at a temperature higher than the alloying temperature between the coating and the steel of the substrate. We thus limit the growth of the ternary alloy which develops between the substrate and the surface layer.

The Applicant has realized that the aluminized sheet obtained with this process not only presents a total emissivity more weak than that of a usual aluminized sheet, such as from the first process step, but also a monochromatic emissivity substantially equal for all wavelengths between 1.5 and 15 micrometers.

This characteristic is perfectly visible in Figure 1 which represents the spectral emissivity of an aluminized sheet B according to the invention, and of an aluminized sheet A of the state of the art.

The first curve, representing the spectral emissivity of a aluminized sheet A of the prior art, was produced from a sheet aluminized made of a titanium IF steel substrate with a thickness equal to 0.3 mm, coated with a layer of a coating comprising 9.5% silicon, 3% of iron, the rest being aluminum, thickness equal to 20 micrometers.

We measured the emissivity of this aluminized sheet for all wavelengths between 1.3 and 15 micrometers, which corresponds at wavelengths characteristic of infrared.

As can be seen, the monochromatic emissivity of this sheet is greater than 0.35 for wavelengths between 2 and 3.6 micrometers, and is less than 0.15 only for wavelengths greater than 7.5 micrometers, while remaining greater than 0.07.

Thus a heat shield made from such a sheet aluminized will be perfectly suited to isolate sources whose energy maximum emission radiative concerns wavelengths greater than 7.5 micrometers, corresponding to the gray bodies to which we can assimilate the exhaust lines at temperatures below 500 ° C.

However, the heat shield effect will be degraded in the case sources with emitted wavelengths less than 7.5 micrometers, corresponding for exhaust lines to temperatures above 500 ° C, i.e. the hottest such as for example the catalyst.

The second curve, representing the spectral emissivity of a aluminized sheet according to the invention (B), was produced from a sheet aluminized made of a titanium IF steel substrate with a thickness equal to 0.3 mm, coated with a layer of a coating comprising 9.5% silicon, 3% of iron, the rest being aluminum, thickness equal to 20 micrometers. This aluminized sheet, cooled to room temperature, has undergone a reheating to 600 ° C, maintaining at this temperature for 5 seconds, then natural cooling to room temperature.

We also measured the emissivity of this aluminized sheet for all wavelengths between 1.3 and 15 micrometers.

As can be seen, the monochromatic emissivity of this aluminized sheet according to the invention is less than 0.15 for all wavelengths between 1.5 and 15 micrometers, and more precisely between 0.10 and 0.15 for wavelengths between 1.5 and 4.5, between 0.07 and 0.10 for lengths waves between 4.5 and 6.5, and less than 0.7 for the lengths waves greater than 6.5.

Thus a heat shield made from such a sheet aluminized according to the invention will be perfectly suited to isolate from sources the maximum emission radiative energy of which concerns wavelengths between 1.5 and 15 micrometers, i.e. for the entire spectrum corresponding to infrared.

Such an aluminized sheet according to the invention is therefore perfectly suitable for making heat shields, whatever the temperature reached by the thermal source to be isolated, and therefore in the case of lines exhaust for all parts of such a line, even the most hot.

This aluminized sheet according to the invention has in terms emissivity, values barely higher than that of aluminum, greater on the order of 0.02 to 0.03 for the wavelengths included between 5.5 and 15 micrometers, and higher on the order of 0.03 to 0.05 for wavelengths between 1.5 and 5.5 micrometers.

Claims (12)

1. Steel sheet coated on at least one of its main faces with a layer of a coating consisting of an aluminium-based alloy, of the type containing aluminium and silicon, with a percentage by weight of less than 11% silicon, characterized in that the coated face has a monochromatic emissivity of less than 0.15 for all wavelengths between 1.5 and 15 micrometres.
2. Coated steel sheet according to Claim 1, characterized in that the coated face has a monochromatic emissivity of less than 0.10 for all wavelengths between 5 and 15 micrometres and a monochromatic emissivity of between 0.10 and 0.15 for all wavelengths between 1.5 and 5 micrometres.
3. Coated steel sheet according to either of the preceding claims, characterized in that the coating layer consists of an aluminium-based alloy containing, in percentages by weight, between 7 and 11% silicon and between 87 and 93% aluminium.
4. Process for manufacturing a steel sheet coated on at least one of its main faces with a layer of a coating consisting of an aluminium-based alloy, of the type containing aluminium and silicon, with a percentage by weight of less than 11% silicon, characterized in that it comprises the following steps:

   production of a steel sheet coated on at least one of its main faces with a layer of a coating in the solid state, consisting of an aluminium-based alloy, of the type containing aluminium and silicon, with a percentage by weight of less than 11% silicon;

   heating of the coating layer up to a temperature (T1) above the melting point (T2) of the said coating;

   holding the coating layer at this temperature level above the melting point T2 of the coating for a time of between 0 and 100 seconds, preferably between 0 and 10 seconds;

   cooling the sheet down to a temperature at least equal to the temperature of the end of alloying between the coating and the steel, preferably down to room temperature.
5. Process according to Claim 4, characterized in that the heating temperature (T1) is between the melting point (T2) of the coating layer and 650°C.
6. Process according to Claim 4 or 5, characterized in that the heating temperature (T1) is greater than the melting point (T2) of the coating layer by 10 to 15°C.
7. Process according to Claim 4, 5 or 6, characterized in that the coating layer is heated at a rate of between 20 and 100°C per second.
8. Process according to Claim 4, characterized in that the cooling of the sheet is natural cooling in the open air, or forced cooling by radiation.
9. Process according to Claim 4, characterized in that the cooling of the sheet is forced air cooling.
10. Process according to Claim 4, characterized in that the cooling of the sheet is carried out in at least two steps, comprising:

    natural cooling down to the melting point of the coating;

    then forced air cooling down to the temperature of the end of alloying between the coating and the steel.
11. Process according to Claim 4, characterized in that the steel sheet coated on at least one of its main faces with a layer of a coating in the solid state, consisting of an aluminium-based alloy, of the type containing aluminium and silicon, with percentages by weight of less than 11% silicon, is produced by dipping a steel substrate into a molten bath containing between 9 and 10% silicon, approximately 3% iron and the balance being aluminium, and cooling down to a temperature below the melting point (T2) of the coating.
12. Heat shield, characterized in that it is made from a sheet blank according to one of Claims 1 to 3.

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